Split seal

ABSTRACT

A split seal (1) having an annular cross-section and a central axis (X) comprises at least two components (2,3) of arcuate form, each component having two end portions (4,4&#39;,5,5&#39;) and each end portion having a surface (F1,F2) formed and arranged for mating engagement with a respective complementary end portion surface of another of the components. Each end portion surface (F1,F2) extends in an indirect manner over one of the radial width (δ r ) and the axial length (l) of the seal and each end portion surface (F1,F2) extends substantially continuously in non-reentrant manner over the other one of the radial width (δ r ) and the axial length (l) of the seal so that the matingly engaged components are substantially smoothly, relatively translatable in one of a generally axial direction and a generally radial direction. In a preferred embodiment each end portion surface (F1,F2) extends in a zigzag manner.

This invention relates to split seals for use in sealing arrangementsfor rotating, stationary and reciprocating elements such as, forexample, positive displacement pumps and hydraulic rams or pistons, andother valve-actuated applications. Such elements are used in systemswithin a wide range of technical fields including the petro chemical,drilling, mining and oil industries, as well as in quarrying and powerengineering fields.

One known type of split seal is formed by breaking an integral ring ofmaterial (commonly carbon) in half to form two components. In use, thetwo components are placed together again around, for example, a rotatingshaft. The components must, however, be placed together very carefullyduring installation in order to avoid damage to the contact facesbetween the components. Such damage can result in the creation of aradial or axial leakage path across the contact faces of the seal,leading to poor and/or inadequate performance of the seal. Additionally,the two halves may be damaged on installation such that the two halvescannot mate perfectly, leading to ineffective sealing due to damage tothe contact faces. Furthermore, damage to the contact faces can occurduring handling of the seal which can render the seal ineffective. Afurther disadvantage of such seals is that grooves are cut on one axialend face of the seal, and axially on a radially inner surface of theseal, during manufacture in order to create weak points to aid breakingof the seal in two. Such grooves can allow undesirable build up ofpressure at the contact faces of the seal during use which, in turn, canlead to fluid leakage in the radial direction. Fluid leaking in betweenthe contact faces acts like a "wedge" between the two seal halves,pushing the two halves apart as the pressure increases.

Other known types of annular split seal rely on the mating of opposedplanar faces which extend radially of the seal and which comprise or arecoated in a hardened material such as ceramic or tungsten carbide. Whilesuch materials have a certain resilience, providing some limitedprotection against damage, each contact face is essentially a singleplane surface, so any damage which does occur during installation orhandling may readily destroy the integrity of the seal. Additionally, inuse, externally applied radially inward mechanical pressure is requiredto maintain the contact faces in engagement.

The conventional "broken" split seals also have the disadvantage ofnecessitating substantial maintenance "down time" of equipment in whichthe seals are incorporated, due to the need to dismantle majorcomponents of the equipment to allow replacement of worn or damagedseals.

It is an object of the present invention to avoid or minimise one ormore of the above disadvantages.

According to a first aspect of the present invention a split seal havingan annular cross-section and a central axis, and having a radial widthand an axial length, comprises at least two components of arcuate form,each component having two end portions and each end portion having asurface formed and arranged for mating engagement with a respectivecomplementary end portion surface of another said component, in whicheach end portion surface extends in an indirect manner over one of theradial width and axial length of the seal, and which end portion surfaceextends substantially continuously in non-reentrant manner over theother one of the radial width and the axial length of the seal so thatthe matingly engaged components are substantially smoothly, relativelytranslatable in one of a generally axial direction and a generallyradial direction.

Preferably, a substantial extent of each said end portion surfaceextending indirectly over one of the radial width and axial length ofthe seal is inclined at an angle thereto. Each said end portion surfacemay extend in a zigzag manner.

According to a second aspect of the present invention a split sealhaving an annular cross-section and a central axis comprises at leasttwo components of arcuate form, each component having two end portionsand each end portion having a surface formed and arranged for matingengagement with a respective complementary end portion of another saidcomponent, in which each end portion surface extends, in the plane ofthe annular cross-secton of the seal, at at least one portion betweenthe radially inner and radially outer faces of the seal, at an angle to,and intersects, an outer radius of the seal and which end portionsurface extends substantially continuously in non-reentrant manneraxially of the seal so that the matingly engaged components aresubstantially smoothly, relatively translatable in a direction generallyaxially of the seal for assembly of the split seal around a shaft in usethereof.

In use of the split seal according to the second aspect of theinvention, one or more angled surface portions of at least one endportion of each component may bear radially against one or morecorrespondingly angled surface portions of a respective complementaryend portion(s) of another of said components due to centrifugal forceacting radially outwardly on fluid which is trapped between a radiallyinner surface of the seal and a rotating shaft or other like elementwhich the seal surrounds. This radial pressure acting between thecomponents maintains the components in mating engagement and provides aneffective radial fluid seal at the splits in the seal where thecomponents engage with one another. This has the advantage of counteringany tendency for the fluid to enter the splits between the engaging endportions of the components of the seal, thus preventing, or at leastminimising, radial fluid leakage through the seal splits. This, in turn,enables the seal to operate at higher fluid pressures than the knownconventional split seals.

Additionally, as the matingly engaged components are substantiallysmoothly, relatively translatable in a direction generally axially ofthe seal, installation may be carried out using generally axial assemblytechniques, thus removing the radial assembly restrictions imposed wheninstalling seals formed by the breaking technique, which type of sealsare easily damaged by friction during any relative axial translation ofthe engaged components that may inadvertently occur.

According to a third aspect of the present invention a split seal havingan annular cross-section and a central axis comprises at least twocomponents of arcuate form, each component having two end portions andeach end portion having a surface formed and arranged for matingengagement with a respective complementary end portion surface ofanother said component, in which each end portion surface extends at atleast one portion between the first and second axial end faces of theseal at an angle to the central axis of the seal and which end portionsurface extends substantially continuously in non-reentrant mannerradially of the seal so that the matingly engaged components aresubstantially smoothly, relatively translatable in a direction generallyradially of the seal.

The axial pressure acting between the components maintains thecomponents in mating engagement and provides an effective axial fluidseal at the splits in the seal where the components engage with oneanother. This counters the tendency for fluid to enter the splitsbetween the engaging end portions of the seal and enables the seal tooperate at higher fluid pressures than known conventional split sealswhen used in axial fluid pressure applications.

The split seal according to the third aspect of the invention may beused in many different industrial fields in place of conventional typesof lip seals and valve stem seals which are traditionally completelyformed (one-piece) rings of various types and materials as required bythe particular application concerned.

In the context of the present invention, the term "substantiallysmoothly" means that the matingly engaged components are relativelytranslatable while in substantially close interengagement withoutsubstantial axial force being exerted on one or more of the componentsin order to overcome resistance to relative translation between thecomponents which might, for example, be presented by irregularities inany of the engaging surfaces of the components. Consequently, thecomponents of the seal according to the second and third aspects of thepresent invention can be relatively translated in the generally axial orradial direction of the seal respectively substantially without causingerosion or damage to the interengaging surfaces of the components whichcould lead to radial or axial fluid leakage respectively between theadjoining end portion surfaces in the seal.

The relative translation of two matingly engaged components generallyaxially of the seal in accordance with the second aspect of theinvention and generally radially in accordance with the third aspect maybe achieved by substantially rectilinear movement of one componentrelative to the other component(s). Where the relative translation isgenerally axially of the seal, relative translation may however also beachieved by movement including a greater or lesser rotational componenti.e. a generally helical movement of one component relative to theother(s).

In the split seal according to the second aspect of the invention eachend portion of each component may be provided with a series ofdifferently angled surface portions, each surface portion extending, inthe plane of the annular cross-section of the seal, at an angle to, andintersecting, an outer radius of the seal. The series of differentlyangled surface portions may comprise pairs of oppositely angled surfaceportions. The oppositely angled surface portions may be defined by anundulating surface provided on each component end portion. Preferably,each end portion of each component comprises a toothed, preferably agenerally saw-toothed, configuration. The tips of the teeth may beradiussed, preferably only to a small degree. This may increase thestrength of the seal.

In use, similarly angled surface portions provided on one end portion ofeach component may each bear against a respective similarly angledsurface portion provided on the respective complementary end portion ofanother component. In the split seal according to the second aspect ofthe invention, similarly angled surface portions on one end portion ofeach component may bear radially outwardly against respective similarlyangled surface portions provided on the respective complementary endportion of another component. Such an arrangement presents alabyrinth-like path to any fluid which enters the "splits" in the seal,thus eliminating or minimising radial fluid leakage. In the split sealaccording to the third aspect of the invention, similarly angled surfaceportions on one end portion of each component may bear axially upwardlyor downwardly against respective similarly angled surface portionsprovided on the respective complementary end portion of anothercomponent. This arrangement eliminates or minimises axial fluid leakage.Additionally, in either case, any damage which occurs to any one angledsurface portion of any component during handling or installation of theseal will have only limited effect on the operation and integrity of theseal since the angles at which the one or more angled surface portionsof each component end portion are disposed are preferably pre-determinedduring manufacture of the seal. Preferably, the two end portions of eachcomponent are complementary. The components are thus interchangeable andcan be replaced by other components of the same dimension. This allowsfor easy replacement of a worn or damaged component without requiringreplacement of the complete seal.

The seal according to the second aspect of the invention may be used inapplications where an element which it surrounds is rotating in aclockwise or an anti-clockwise direction.

The seal may also be used in applications where it surrounds arotationally reciprocating element, and in stationary applications wherehydrodynamic pressure may act radially on the seal. At least one surfaceportion which extends at an angle to, and intersects, an outer radius ofthe seal may be provided on each end portion of each component adjacentto an inner annular surface of the seal. Preferably, these angledsurface portions adjacent to the inner annular surface of the seal leadrearwardly away from the direction of rotation of the rotating elementwhich is surrounded by the seal. By installing the seal in either anupright or an inverted orientation, the seal may be advantageouslyadapted to suit applications in which the seal surrounds an elementwhich is rotating in a clockwise direction or an element which isrotating in an anti-clockwise direction. Fluid on the rotating elementwhich enters the region between the seal and the rotating element may beeffectively restricted from entering the "splits" in the seal (i.e.between the engaging surfaces of the seal) by installing the seal in anorientation in which the angled surface portions provided adjacent tothe inner surface of the seal are directed away from the direction ofrotation of the rotating element, and hence are directed away from fluidcarried on the outer surface of the rotating element.

The seal according to the third aspect of the invention is preferablyused in applications where an element which it surrounds is moving backand forth in a generally axial direction relative to the seal. The sealmay also be used in stationary applications where fluid pressure actsgenerally axially on the first or second axial end faces of the seal.

The angled surface portion(s) are preferably cut into each component ofthe seal during manufacture. Where a plurality of angled surfaceportions are provided on each component end portion, these may beprovided by angled grooves cut into the seal. The angle of each groovemay be in the range of fifty to seventy degrees. Preferably the angle ofeach groove is substantially sixty degrees such that the acute anglebetween each angled surface portion and the radius is substantiallysixty degrees.

The seal is preferably formed of a relatively easily worked metal suchas, for example, aluminium. Alternatively the seal or the individualseal components may be injection molded or cast. Other suitablematerials for the seal are those currently used as sealing materialswithin the sealing industry, in particular resin impregnated carbon. Thesurfaces of the seal components may be coated with a resilient material.This provides the seal with an overall resilient surface applicable tomost uses within various industries. Cutting the required angled surfaceportion(s) in the end portions of each component avoids the need for thecreation of weak points or grooves to aid breaking of the seal duringmanufacture, as required with some conventional split seals.

According to a fourth aspect, the invention comprises a component forincorporation in a split seal of annular cross section having a centralaxis, the component being of arcuate form and comprising two endportions and each end portion having a surface formed and arranged formating engagement with a respective complementary end portion surface ofanother such component, in which each end portion surface extends, inthe plane of the annular cross-section of the seal, at at least oneportion between the radially inner and radially outer faces of the seal,at an angle to, and intersects, an outer radius of the seal, and whichsurface extends substantially continuously in non-reentrant manneraxially of the seal so that the component is substantially smoothlytranslatable with respect to the other component, when the componentsare matingly engaged, in a direction generally axially of the seal.

According to a fifth aspect, the invention comprises a component forincorporation in a split seal of annular cross-section having a centralaxis, the component being of arcuate form and comprising two endportions and each end portion having a surface formed and arranged formating engagement with a respective complementary end portion surface ofanother such component, in which each end portion surface extends at atleast one portion between the first and second axial end faces of theseal, at an angle to the central axis of the seal, and which surfaceextends substantially continuously in non-reentrant manner radially ofthe seal so that the component is substantially smoothly translatablewith respect to the other component, when the components are matinglyengaged, in a direction generally radially of the seal.

Embodiments of the invention will now be described by way of exampleonly, with reference to the accompanying drawings, in which:

FIG. 1 is a top view of a split seal according to the invention;

FIG. 2a is a perspective view of one component of the seal of FIG. 1;

FIG. 2b is a fragmentary top view of the split seal of FIG. 1;

FIG. 3a is a cross-sectional side view of a shaft and housing assemblyincorporating two conventional split seals;

FIG. 3b is a perspective fragmentary view of the assembly of FIG. 1;

FIG. 4a is an exploded fragmentary top view of a split seal according tothe invention illustrating centrifugal force acting on angled contactareas of surface areas of one end portion of each component of the seal,where the seal surrounds a shaft (not shown) rotating clockwise;

FIG. 4b is an exploded fragmentary top view of the inverted split sealof FIG. 4a indicating centrifugal force acting on angled contact surfaceareas of one end portion of each component of the seal, where the shaftis rotating anti-clockwise.

FIG. 5a is a top view of an alternative embodiment of a split sealaccording to the invention, surrounding a piston;

FIG. 5b is a left hand side elevation of the seal of FIG. 5a; and

FIG. 5c is a right hand side elevation of the seal of FIG. 5a.

FIG. 1 shows a split seal 1 of generally cylindrical form having anannular cross-section and a central axis X and comprising two components2, 3 of arcuate form. The seal 1 has a radial width δ_(r) and an axiallength l. The seal 1 has a radially inner face 20 and a radially outerface 22. Each component comprises a first end portion 4, 4' and a secondend portion 5, 5'. The two components are identical and one component 2is shown in FIG. 2a. Each end portion 4, 5 of that component 2 comprisesa contact face F1, F2 defined by a series of teeth or leading edges 8formed by grooves which are cut into that component 2 duringmanufacture. The two end portions of that component 2 are complementarysuch that the teeth of each end portion 4, 5 can be arranged in matingengagement with the teeth of the complementary end portion 5', 4' of theother component 3 of the seal so as to form the assembled split seal 1shown in FIG. 1. The toothed configuration of the end portions providesa plurality of discrete oppositely angled surfaces 6, 7 in each contactface F1, F2 of the end portions of the two components, 2, 3. As shown inFIG. 2b, each surface 6, 7 extends, in the plane of the annularcross-section of the seal, at an acute angle Q of sixty degrees to, andintersects, an outer radius R of the seal. Each contact face F1, F2extends substantially continuously in non-reentrant manner axially ofthe seal.

FIGS. 3a, 3b illustrate a conventional arrangement incorporating twoconventional split seals. One seal 9 is mounted in a housing 15 andsurrounds a shaft or piston 11 located on the axis X of the seal. Asecond seal 10, similar to the first, is fixed to the shaft (byclamping, bolting or otherwise) so as to rotate with the shaft. Thearrangement of the two seals is such that one axial face 12 of the sealfixed to the shaft abuts an axial face 14 of the seal mounted in thehousing, the seal 9 mounted in the housing remaining stationary whilethe seal 10, which is fixed to the shaft rotates with the shaft 11. Aseal 1 according to the present invention may be used in place of eitheror each of the conventional seals 9, 10.

During installation of the split seal 1, the two components cannot beinstalled wrongly as their configuration only allows them to be mated inone way to form the annular seal 1. The contact faces F1, F2 can slidesmoothly across one another in a generally axial direction duringassembly of the seal without causing damage to the angled surfaces 6, 7.

In use, the fluid enters the region between the inner annular surface ofeach seal and the outer surface of the shaft. Fluid may also enter theregion between the abutting axial faces of the seals where it acts as alubricant before being vapourised by heat produced by friction betweenthe axial faces of the seal. Rotation of the shaft creates hydrodynamicpressure between each seal and the shaft 11 due to centrifugal forceacting radially on the fluid trapped between the seals and the shaft.This pressure acts radially on certain of the angled surfaces 6, 7 ofthe teeth 8 to press the components 2, 3 together. Those of the angledsurfaces upon which this radial pressure acts is dependent upon thechosen orientation of the seal. The seal has an upright or an invertedorientation, in use.

FIG. 4a illustrates one orientation of an alternative embodiment of thesplit seal 1, where features corresponding to the seal of FIGS. 1, 2aand 2b are correspondingly numbered, and in which parallel angledsurfaces 6 of one end portion 4, 4' of each component 2, 3 press againstsimilarly angled surfaces 6 of a respective end portion 5', 5 of theother component, in a radially outward direction (indicated by arrow Pc,Pc'). This orientation is particularly suited to the situation where theshaft is rotating in a clockwise direction, indicated by the arrows C(shaft not shown), since the angled surfaces nearest to the innerannular surface 20 of the seal are angled so as to lead rearwardly awayfrom the direction of rotation of fluid carried by the outer surface ofthe rotating shaft. This tends to prevent fluid entering the splits inthe seal.

FIG. 4b illustrates the situation where the shaft is rotating in ananti-clockwise direction, indicated by the arrows A, and the seal 1 isin an inverted orientation. The parallel angled surfaces 6, which arenow oppositely angled (with regard to the first orientation of theseal), of the same end portion 4, 4' of each component 2, 3 again pressagainst the similarly angled surfaces 6 of the respective complementaryend portion 5', 5 of the other component (indicated by arrows Pa, Pa')in a radially outward direction.

In the embodiments shown in FIGS. 4a and 4b, each end portion comprisessix angled surfaces defining the teeth 8. One end portion of eachcomponent 2, 3 comprises four teeth 8 while the other end portioncomprises three teeth. However, fewer or further teeth 8 may be providedon the end portions of alternative embodiments of the invention (see,for example, FIGS. 1 and 2). Similarly, the angle of the angled surfaces6, 7 may be greater or less than sixty degress in alternativeembodiments of the invention (not shown).

The angled surfaces 6, 7 can be measured and the form and dimensions ofthe end portions 4, 5, 4', 5' are thus capable of being accuratelyknown. The number of angled surfaces 6, 7 which are provided on eachcontact face F1, F2 gives a known sealing parameter for the contactfaces of the split seal.

The split seal is manufactured from a ring of relatively easily workablemetal, for example, aluminium, which is cut in half to form two arcuateportions. The teeth 8 are then cut into the end portions of the cuthalves to form the two components 2, 3 of the seal. The two halves arethen fitted together and the seal 1 is machined to the desireddimensions. By machining the two halves this way, a specific desirednumber of individual angled surfaces 6, 7 may be provided.

In an alternative embodiment, the seal 1 or the components 2, 3 areinjection moulded in plastics material.

Instead of manufacturing the end portions with teeth 8 defining thedistinct angled surfaces 6, 7 each end portion may be provided with acontinuous, undulating surface of peaks and troughs incorporating thedesired angled surface portions. In this embodiment, the centrifugalforces act similarly through angled surface portions of one componentend portion against parallel angled surface portions of the respective,matingly engaged component end portion. In another embodiment the tipsof the teeth 8 may be radiused so as to be slightly rounded. Thisincreases the strength of the seal.

The split seal 1 can be used in applications incorporating rotating,reciprocating or stationary elements, such as pumps and hydraulic ramsor pistons, in many industrial applications and particularly in thedrilling industry where the seal can be provided for full bore sealingarrangements. The seal can be manufactured to any desired dimension.Additionally the seal allows for a certain degree of misalignment whichcan be advantageous on large, slow rotating elements operating at lowpressures.

It is believed that the split seal 1 will operate successfully at fluidpressures of up to 20 bar (290 p.s.i.) and over.

The seal components 2, 3 can be coated with a suitable material, such asa ceramic material, to give an overall resilient surface applicable tomost uses within various industries. The magnitude of the inner radiusof the seal can be chosen to give a desired clearance between the sealand the shaft to control the quantity of fluid trapped between the sealsand the shaft or piston, which in turn provides greater or lesserlubrication between the axial faces of the two seals 9, 10.

The seal components 2, 3 can be utilised as consumables and can beinterchanged, thus minimising machinery "down time" for normal routinemaintenance and/or breakdowns where the necessity to strip majorcomponents for complete ring split seal replacement is not necessary.

Critical tests have found the seal to be potentially effective in termsof pressure containment at fluid pressures of 2 Bar (29 p.s.i.) held fora duration of 20 minutes whith a shaft rotation speed of 500 r.p.m.Sample pressure containment of the seal at 95 bar (1378 p.s.i.) in astatic test mode has been witnessed and verified by Bureau Veritas.Other tests have shown the seal to be effective at up to 15 Bar (218p.s.i.) fluid pressure for 30 minutes at a shaft rotation of 1000 r.p.m.

Another embodiment of the invention is illustrated in FIGS. 5(a), 5(b),5(c). In this embodiment as in the embodiments of FIGS. 1 to 4, a splitseal 1 of generally cylindrical form having an annular cross-section anda central axis X comprises two components 2,3 of arcuate form. The seal1 has a radial width δ_(r) and an axial length l. The two components areidentical and each has a first end portion 4,4' and a second end portionof 5,5'. As in the split seal shown in FIGS. 1 to 4, each end portion ofthe seal 1 comprises a contact face F1,F2 defined by a series of teethor leading edges which provide six discrete oppositely angled surfaces6',7' in each contact face F1,F2. The teeth of each end portion 4,5 ofone component are arranged in mating engagement with the teeth of thecomplemetary end portion of the other component 3 of the seal so as toform the assembled split seal as in FIG. 5a, where the seal 1 is shownassembled round a piston or other element 30. The piston is located forreciprocal transitional movement along the axis X in this embodiment ofthe seal. In other applications, the element 30 may be static. As shownin FIGS. 5(b) and (c), each of the angled surfaces 6',7' extends at anangle Z of 60 degrees to the central axis X of the seal. Each contactface F1,F2 extends substantially continuously in non-reentrant mannerradially of the seal 1. The contact faces F1,F2 can slide smoothlyacross one another in a generally radial direction without causingdamage to the angled surfaces 6',7'. As the components 2,3 are identicalthey can thus be interchanged, thereby eliminating the need for"complete ring" seal replacement which in the past has requireddismantling of major components and subsequent "down-time" of equipment.

The seal 1 of FIGS. 5(a), 5(b) and 5(c) operates in a similar manner tothat of FIGS. 1 to 4, the main difference being that the similarlyangled surfaces of the matingly engaged end portions of the components2,3 bear axially against each other under fluid pressures actinggenerally axially on the upper 32 or lower 34 axial end surfaces of theseal, in use of the seal. The contact faces F1,F2 present alabyrinth-like path to any fluid attempting to enter the seal betweenthe contact faces F1,F2.

The seal of FIGS. 5(a), (b) and (c) has been found to performeffectively in terms of pressure containment at fluid pressures up to150 Bar in a static test mode (i.e. where the piston 30 or other elementwhich the seal 1 surrounds is static).

What is claimed is:
 1. A split seal having an annular cross-section anda central axis, and having a radial width and an axial length,comprising at least two components of arcuate form, each componenthaving two end portions and each end portion having a surface formed andarranged for mating engagement with a respective complementary endportion surface of another said component, wherein each end portionsurface comprises a plurality of angled surface portions extending insubstantially zigzag manner over one of the radial width and the axiallength of the seal, each angled surface portion intersecting a commonaxial plane of the seal, and each end portion surface extendingsubstantially continuously in non-reentrant manner over the other one ofthe radial width and the axial length of the seal, so that thecomponents are substantially smoothly, relatively translatable in agenerally radial direction or a generally axial direction, respectively.2. The split seal according to claim 1, wherein two said matinglyengaged components are relatively translatable by substantiallyrectilinear movement of one component relative to the other component.3. The split seal according to claim 1, wherein two said matinglyengaged components are relatively translatable by movement including arotational component of movement.
 4. The split seal according to claim 3wherein two said matingly engaged components are relatively translatableby substantially helical movement of one component relative to theother.
 5. The split seal according to claim 1 wherein said plurality ofangled surface portions extend in substantially zigzag manner in theplane of the annular cross-section of the seal, between the radiallyinner and radially outer faces of the seal, each angled surface portionextending, in the plane of the annular cross-section of the seal, at anangle to, and intersecting, an outer radius of the seal, and each endportion surface of the seal extends substantially continuously innon-reentrant manner axially of the seal so that the matingly engagedcomponents are substantially smoothly, relatively translatable in adirection generally axially of the seal for assembly of the split sealaround a shaft in use thereof.
 6. The split seal according to claim 5,wherein at least one surface portion which extends at an angle to, andintersects, an outer radius of the seal is provided on each end portionof each seal adjacent to a radially inner annular surface of the seal,said surface portions of each seal component adjacent the radially innerannular surface of the seal being formed and arranged so as to leadrearwardly away from a direction of rotation of a rotating elementsurrounded by the seal in use thereof.
 7. The split seal according toclaim 1, wherein said plurality of angled surface portions extend insubstantially zigzag manner, between the first and second axial endfaces of the seal, each angled surface portion extending at an angle tothe central axis of the seal.
 8. The split seal according to claim 1,wherein said plurality of angled surface portions comprises pairs ofoppositely angled surface portions.
 9. The split seal according to anypreceding claim, wherein each end portion of each component comprises asaw-toothed configuration.
 10. A split seal according to claim 9 whereinthe teeth of the saw-toothed configuration have radiused tips.
 11. Thesplit seal according to claim 1, wherein said plurality of angledsurface portions are provided in the form of angled grooves cut into theseal.
 12. The split seal according to claim 11 wherein of each groove isin the range of fifty to seventy degrees.
 13. The split seal accordingto claim 11 wherein the angle of each groove is substantially sixtydegrees.
 14. The split seal according to claim 1, wherein the two endportions of each component are complementary whereby said components areinterchangeable and replaceable by like components.
 15. The split sealaccording to claim 1, wherein the seal is machined from aluminum. 16.The split seal according to claim 1, wherein said components of the sealare injection molded in plastics material.
 17. The split seal accordingto claim 1, wherein said components of the seal are formed ofresin-impregnated carbon.
 18. The split seal according to claim 1,wherein the surfaces of said components of the seal are coated with aresilient material.
 19. A component for incorporation in a split seal ofannular cross-section having a central axis, the component being ofarcuate form and comprising two end portions and each end portion havinga surface formed and arranged for mating engagement with a respectivecomplementary end portion surface of another such component, whereineach end portion surface comprises a plurality of angled surfaceportions extending in substantially zigzag manner, in the plane of theannular cross-section of the seal, between radially inner and radiallyouter faces of the seal, each angled surface portion intersecting acommon axial plane of the seal, and each end portion surface extendingsubstantially continuously in non-reentrant manner axially of the sealso that the component is substantially smoothly translatable withrespect to the other component, when the components are matinglyengaged, in a direction generally axially of the seal.
 20. A componentfor incorporation in a split seal of annular cross-section having acentral axis, the component being of arcuate form and comprising two endportions and each end portion having a surface formed and arranged formating engagement with a respective complementary end portion surface ofanother such component, wherein each end portion surface comprises aplurality of angled surface portions extending in substantially zigzagmanner, between first and second axial end faces of the seal, eachangled surface portion intersecting a common axial plane of the seal,and each end portion surface extending substantially continously innon-reentrant manner radially of the seal so that the component issubstantially smoothly translatable with respect to the other component,when the components are matingly engaged, in a direction generallyradially of the seal.
 21. A split seal having an annular cross-sectionand a central axis comprises at least two components of arcuate form,each component having two end portions and each end portion having asurface formed and arranged for mating engagement with a respectivecomplementary end portion surface of another said component, in whicheach end portion surface extends in an indirect manner, in the plane ofthe annular cross-section of the seal, at at least one portion betweenthe radially inner and radially outer faces of the seal, which endportion surface extends substantially continuously in non-reentrantmanner axially of the seal so that the matingly engaged components aresubstantially smoothly, relatively translatable in a direction generallyaxially of the seal for assembly of the split seal around a shaft in usethereof, characterised in that at least one surface portion whichextends at an angle to, and intersects, an outer radius of the seal isprovided on each end portion of each seal component, adjacent to aradially inner annular surface of the seal, said surface portions ofeach seal component adjacent the radially inner annular surface of theseal being formed and arranged so as to lead away from a direction ofrotation of a rotating element surrounded by the seal in use thereof.22. The split seal according to claim 21 wherein a substantial extent ofeach said end portion surface extending in an indirect manner isinclined at an angle to the outer radius of the seal.
 23. The split sealaccording to claim 21, wherein each said end portion surface extends inzigzag manner.